The Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide is used as a probe molecule and systems are evaluated by 31P NMR spectroscopy. Gutmann used 31P NMR spectroscopy to parameterize Lewis acidity of solvents by Acceptor Numbers. Beckett recognised its more generally utility and adapted the procedure so that it could be easily applied to molecular species when dissolved in weakly Lewis acidic solvents. However, it has been shown that there is no one universal order of Lewis acidstrengths and that two parameters or two properties are needed to define acid and base strengths. Single parameter or property scales are limited to a smaller range of acids or bases. The term Gutmann–Beckett method was first used in chemical literature in 2007 and the Gutmann-Beckett method is in current use due to its experimental convenience. Prof. Dr Viktor Gutmann was an eminent Austrian chemist renowned for his work on non-aqueous solvents. Prof. Michael A. Beckett is a former Head of the School of Chemistry at Bangor University, UK.
The 31P chemical shift of Et3PO is sensitive to chemical environment but can usually be found between +40 and +100 ppm. The O atom in Et3PO is a Lewis base, and its interaction with Lewis acid sites causes deshielding of the adjacent P atom. Gutmann described an Acceptor Number scale for solvent Lewis acidity with two reference points relating to the 31P NMR chemical shift of Et3PO in the weakly Lewis acidic solvent hexane and in the strongly Lewis acidic solvent SbCl5. Acceptor numbers can be calculated from AN = 2.21 x and higher AN values indicate greater Lewis acidity. Boron trihalides are archetypalLewis acids and have the following AN values: BF3 < BCl3 < BBr3 < BI3. The Lewis acidity of other molecules can be obtained in weakly Lewis acidic solvents by 31P NMR measurements of their Et3PO adducts. The Gutmann–Beckett method has been applied to Lewis acids derived fluoroarylboranes such as B3, and borenium cations, and its application to a variety of boron compounds has been reviewed.
Application to other compounds
The Gutmann–Beckett method has been successfully applied to alkaline earth metalcomplexes, p-block main group compounds 2]3+ and transition-metal compounds .
